Many currently deployed cellular telephone networks rely upon Wideband Code Division Multiple Access (WCDMA) technology to transmit information over the air interface. These include Universal Mobile Telecommunications Systems (UMTS) based 3G networks. Whilst it is anticipated that at some point in the future WCDMA networks will be replaced with so-called 4G networks, there is an ongoing process to improve the performance of deployed networks until such time as 4G networks are introduced. FIG. 1 illustrates a number of components of a WCDMA network including, on the network side, a Radio Base Station (RBS) 1 and a Radio Network Controller (RNC) 2, as well as a user terminal or User Equipment (UE) 3.
One improvement that is being considered is the use of a technique known as Interference Cancellation (IC). IC might be used in order to achieve better performance in terms of, for example, peak data rates, system throughput and system capacity. IC is applied in the uplink direction from the UE to the RBS (and also potentially in the downlink direction). As such, the main impact of IC is felt at the RBS acting as receiver of the uplink channels. In the uplink direction, IC relies upon the fact that data transmitted by certain users (the “cancellers”) is more “visible” within a received interference signal than data transmitted by other users (the “cancellees”). The basic steps in an IC process (at the RBS) are as follows:                1. detecting data in the signals sent by the cancellers,        2. regenerating an interference signal at the receiver using the detected data,        3. cancelling the regenerated interference signal from the received signal, and        4. detecting the signals of the remaining users, i.e. the cancellees.        
This process is illustrated schematically in FIG. 2. The first step can be done before decoding (pre-decoding IC) or after decoding (post-decoding IC), where decoding is the process of removing the extra coding applied at the sender (often called channel coding). Channel coding involves adding extra bits to protect the information bits from errors, i.e. these extra bits can be used to correct the information bits if they have been corrupted during the radio transmission. The second step in the process involves mimicking how the transmitted bits arrive at the receiver. This involves going through the operations performed at the transmitter (what the transmitter has done to the data bits) and channel filtering (what the channel has done to the data bits).
The IC approach described above introduces significant delays in the layer 1 control loops, e.g. fast power control and HS-signalling. The detection and regeneration of the interference signal causes significant delays before the cancellees are treated. Among other control loops, the High Speed Downlink Packet Access (HSDPA) signalling on High Speed-Dedicated Physical Control Channel (HS-DPCCH) in the uplink, i.e. the HARQ ACK/NACK and the CQI signalling for the cancellee, will be adversely affected by this delay.
If the HS-DPCCH channels for the cancellees are detected after IC has been performed, then the increased delay for handling the ACK/NACK messages for the cancellees will cause extra HARQ processes to be used in the downlink. The number of processes running concurrently determines the number of “packets” that can be in-flight at any given time. As there are a limited number of such processes, and there might not be more processes available, a delay in handling the ACK/NACK messages for the cancellees at the RBS will cause the downlink data flow to wait until an in-use HARQ process is freed. The result of the extra delay will be a lower data throughput in the downlink. Furthermore, if the available memory allocated for the soft buffer (the soft buffer is an area of memory used to store data from erroneous data frames and which can be used to process subsequently received frames)—the size of which is standardized in 3GPP—is allocated to an increased number of HARQ processes, the largest transport blocks may no longer be supported and the performance of the largest supported transport blocks will decrease. Simulations indicate a performance loss exceeding 3 dB. As a consequence, both the supported theoretical maximum data rate and the achieved data rate in a realistic scenario would be impacted negatively.
If on the other hand the HS-DPCCH channels for the cancellees are detected prior to IC (thus avoiding the extra delays), the lower signal quality will require an increase of the HS-DPCCH transmitted power from the respective UEs, causing extra cell load and lower uplink cell capacity. If the HS-DPCCH channel transmitted power is not increased, then the lower signal quality will result in reduced capacity in the downlink for HSDPA.
Increasing the HS-DPCCH channel power in all or selected cells (by, for example, increasing the HS-DPCCH channel power offset, i.e. the beta factor, when IC is applied) is very expensive in terms of overall cell capacity, since the increased power needs to be applied even if there are no users to be cancelled. Furthermore, there may be cells where IC is not applied at all, and in these cells increasing the HS-DPCCH channel power is also unnecessary. Implementing a solution where the HS-DPCCH channel power is dependent upon whether or not there are users in the cell to be cancelled, and/or upon whether or not IC is applied, and at the same time allowing for user mobility, would probably be too complex in practice. In addition, the link quality of the HS-DPCCH channel would differ from the link quality of the other physical channels detected posterior to IC. This would make link control (e.g. transmit power control (TPC)) more difficult since the link control implicitly assumed in WCDMA relies upon the fact that the relative performance of all physical channels from a particular UE is constant. As a consequence, a lower signal quality for the HS-DPCCH channel will result in worse performance on the downlink for HSDPA.